This application claims the priority of German application 102 103 67.4, filed Mar. 8, 2002, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an exhaust-gas purification installation, for purifying an exhaust gas from an internal combustion engine, having an exhaust-gas catalytic converter which is arranged in an exhaust pipe of the internal combustion engine, and a catalytic fuel reformer adapted to generate a hydrogen-containing reformer gas which can be fed to the exhaust pipe on an entry side of the exhaust-gas catalytic converter. A hydrocarbon-containing fuel, which can be used to operate the internal combustion engine, may be fed to the fuel reformer in order to generate the reformer gas. The invention also relates to an exhaust-gas purification method for purifying exhaust gas from an internal combustion engine, using an exhaust-gas purification system having an exhaust pipe with an exhaust-gas catalytic converter arranged in the pipe and a catalytic fuel reformer, in which a fuel which is used to operate the internal combustion engine is fed to the fuel reformer in order to generate a hydrogen-containing reformer gas.
It is known from laid-open European specification EP 0 537 968 A1 to feed hydrogen-containing gas to exhaust gas from a lean-burn internal combustion engine in order to reduce nitrogen oxides. It is possible to achieve a relatively high reduction in the levels of nitrogen oxides using the supplied hydrogen at relatively low temperatures at special denox catalytic converters even when an excess of oxygen (lean exhaust-gas condition) is present. To generate the hydrogen-containing gas, in European specification EP 0 537 968 A1, it is proposed to use a catalytic reformer in which a fuel which is available on board the corresponding motor vehicle is reacted with the exhaust gas from the internal combustion engine, if appropriate with the addition of water, to form a hydrogen-containing reformer gas which is fed to the denox catalytic converter. Heating the reformer to its operating temperature and maintaining this operating temperature are achieved by the quantitative control of the flow of hot exhaust gas to the reformer. However, hot exhaust gas is not available immediately after the internal combustion engine has been started up, and consequently it takes a certain time to heat up the installation which has been described to its operating temperature, and the function of reducing the levels of nitrogen oxides is likewise only available after a certain time.
Furthermore, it is known from European Patent EP 0 621 400 B1 to feed reducing agent to the exhaust gas from an air-compressing internal combustion engine, i.e. an internal combustion engine which is usually operated with excess air, by late injection of fuel. Depending on the injection time, this fuel is used to heat the exhaust-gas after-treatment device by means of catalytic oxidation at an exhaust-gas catalytic converter or as a reaction partner for reducing the levels of nitrogen oxides at a denox catalytic converter. On account of the method by which it is provided, the reducing agent which is made available consists predominantly of hydrocarbons. However, these hydrocarbons are relatively slow to react compared to hydrogen and, consequently, neither the heating of the exhaust-gas after-treatment device nor the catalytic reduction in the levels of nitrogen oxides at low exhaust-gas temperatures is possible.
It is an object of the invention, using a fuel reformer, to provide an exhaust-gas purification installation and an exhaust-gas purification method with an improved exhaust-gas purification function in which improved operation of the fuel reformer is also possible. This object is achieved by way of an exhaust-gas purification installation having an exhaust-gas heater provided so as to heat an exhaust-gas part-stream which is fed to the fuel reformer. This object is also achieved by way of an exhaust-gas purification method including removing an exhaust-gas part-stream from the exhaust pipe, heating the exhaust-gas part-stream, and feeding the heated exhaust-gas part-stream to the fuel reformer.
An exhaust-gas purification installation according to the invention is distinguished by the fact that it has an exhaust-gas heater for heating an exhaust-gas part-stream fed to the fuel reformer. The reformer and the exhaust-gas heater are arranged as separate components in an additional branch outside the internal combustion engine exhaust pipe but on board a motor vehicle associated with the internal combustion engine. The reformer and the exhaust-gas heater can therefore be operated as far as possible independently of the operating state of the internal combustion engine. To generate a hydrogen-containing reformer gas, an exhaust-gas part-stream which is removed from the exhaust pipe of the internal combustion engine at any desired point flows through the reformer. The exhaust-gas part-stream is, at least from time to time and preferably when the reformer is starting up until it reaches its normal operating state, heated by the exhaust-gas heater before being fed to the reformer. As a result, the reformer catalytic converter is heated at the same time. The fuel which is also required to operate the reformer is taken from the normal fuel supply to the internal combustion engine and injected into the heated exhaust gas on the entry side of the reformer where it is, at least for the most part, vaporized. The heating of an exhaust-gas part-stream naturally takes place more quickly and effectively than the heating of the entire exhaust-gas stream, and consequently the reformer is soon ready for operation and able to supply hydrogen-containing reformer gas. At the same time, the outlay on energy is kept at a low level. Compared to the injection of the fuel into the main-gas stream, moreover, the arrangement according to the invention achieves significantly improved homogenization of the mixture fed to the reformer and therefore effective fuel reforming with a high yield of hydrogen.
In one configuration of the invention, the exhaust-gas heater is of electrically heatable design. The electrical energy required for the heating operation may, for example, be taken from the on-board power supply of the motor vehicle in question. The use of electric heating for the exhaust-gas heater allows the exhaust-gas part-stream to be heated very quickly and, moreover, a high degree of flexibility with regard to the shape of the exhaust-gas heater, since electrical heater elements for different outputs and operating voltages are commercially available at low cost in a very wide variety of forms.
In a further configuration of the invention, the exhaust-gas heater is of cylindrical configuration and, in its interior, has a coiled passage, which runs substantially in the axial direction, for guiding the exhaust-gas part-stream. This embodiment of the exhaust-gas heater creates a large heat-transfer surface area and achieves a correspondingly good heat transfer from the electrical heating to the exhaust-gas part-stream. For this purpose, the electrical heating may be provided in such a way that the inner or outer wall, as seen in the radial direction, or both walls of the coiled passage can be heated by one electrical heater element.
In a further configuration of the invention, a high-temperature heat exchanger, by means of which heat is transferred from the reformer gas emerging from the reformer to the exhaust-gas part-stream fed to the reformer, is also provided as part of the exhaust-gas purification installation. In this case, the high-temperature heat exchanger is connected downstream of the reformer in terms of flow and the reformer gas flows through it. Since the reforming process in the reformer preferably takes place at temperatures of above 500xc2x0 C., the reformer gas which leaves the reformer has a relatively high heat content, a large proportion of which can therefore be fed to the charge gas. In this way, the energy balance of the reforming process can be improved and, during normal operation, additional electrical heating of the exhaust-gas part-stream by the exhaust-gas heater can be avoided or kept to a minimum.
In a further configuration of the invention, the exhaust-gas part-stream can be fed to the fuel reformer via the exhaust-gas heater and/or via the high-temperature heat exchanger, as desired. To optionally switch over the path along which the exhaust-gas part-stream which is fed to the reformer is guided, suitable switching means, such as three-way valves or the like, can be provided. The possibility of switching the path over which the gas is guided makes it possible to keep the distance over which the exhaust-gas part-stream is carried and therefore heat losses at a low level. Therefore, the switching preferably takes place as a function of the reformer operation. When the reformer is being run up to its normal operating state, it is preferable for the exhaust-gas part-stream to be guided via the heat of the exhaust-gas heater, and when the reformer is operating normally it is preferable for the exhaust-gas part-stream to be guided only via the high-temperature heat exchanger. There is preferably also provision for the flow to be guided in such a way that the exhaust-gas part-stream flows through the high-temperature heat exchanger and the exhaust-gas heater in succession.
In a further configuration of the invention, an oxidation catalytic converter is arranged in the exhaust pipe of the internal combustion engine. It is preferable for this oxidation catalytic converter to be arranged in the exhaust pipe close to the internal combustion engine. The reformer gas is fed into the exhaust pipe on the entry side of this catalytic converter as required. On account of the high reactivity of the reformer gas, the catalytically assisted oxidation process in this catalytic converter starts at relatively low temperatures. The heat of reaction which is liberated during this reaction heats the catalytic converter and therefore rapidly increases its catalytic activity even with respect to harmful exhaust-gas constituents which are more difficult to oxidize. As a result, for example when the internal combustion engine is warming up, effective exhaust-gas purification can take place at an earlier time, and therefore the overall emissions of pollutants from the internal combustion engine can be reduced.
In a further configuration of the invention, what is known as a denox catalytic converter is arranged in the exhaust pipe of the internal combustion engine. It is known that catalytic converters of this type are able to catalyse a chemical reduction of nitrogen oxides to form harmless nitrogen under oxidizing conditions using reducing agents which are present in the exhaust gas, such as hydrocarbons or hydrogen. Particularly at low temperatures, in this respect hydrogen represents a very effective and selective reducing agent. Therefore, providing the hydrogen-containing reforming gas makes it possible to achieve an effective reduction in the levels of nitrogen oxides even at low exhaust-gas temperatures, in particular in the case of lean-burn internal combustion engines. This means firstly that the reduction in the levels of nitrogen oxides becomes possible at an early time while the internal combustion engine is still warming up. Secondly, it is possible to elect for the denox catalytic converter to be installed away from the internal combustion engine, where the temperature level is correspondingly low, with the result that greater flexibility in terms of the design of the exhaust-gas purification installation is achieved.
In a further configuration of the invention, an oxidation catalytic converter, which is preferably arranged close to the internal combustion engine, and in addition, further downstream, what is known as a denox catalytic converter are arranged in the exhaust pipe of the internal combustion engine. The reformer gas can be fed to the exhaust pipe on the entry side of both catalytic converters. Therefore, the exhaust-gas purification device which is designed in this way can simultaneously achieve both a reduction in the levels of pollutants emitted while the engine is warming up and a reduction in the levels of nitrogen oxides even when the denox catalytic converter is in an unfavourable installation position or when the exhaust-gas temperatures are low. This is advantageous in particular in the case of an internal combustion engine which is operated predominantly under lean-burn conditions, such as a direct-injection spark-ignition engine operated in stratified-charge mode, a diesel engine or a gas turbine, since these internal combustion engines operate with a very high level of efficiency and therefore have a low exhaust-gas temperature. The exhaust-gas purification installation has switching means for distributing the reformer gas as required, in such a manner that the reformer gas can be fed to the oxidation catalytic converter or the denox catalytic converter or both catalytic converters in a controlled quantity as a function of the internal combustion engine operation and/or as a function of the temperature of the catalytic converters.
A method according to the invention is distinguished by the fact that an exhaust-gas part-stream which is removed from the exhaust pipe is heated and fed to the fuel reformer. An advantage in this context is a high heating temperature, with the result that when the reformer is being started up the heated exhaust-gas part-stream for its part rapidly heats the reformer catalytic converter. With this method, the reformer catalytic converter is heated significantly more quickly than if, for example, the reformer itself or the housing of the reformer were to be externally heated or if the entire exhaust-gas stream had to be heated. In this way, the reformer or its catalytic converter rapidly reaches its operating temperature, whereupon fuel can be injected into the heated exhaust-gas part-stream on the entry side of the reformer. When the exhaust-gas part-stream is at a sufficiently high temperature, the injected fuel is completely or almost completely vaporized, with the result that good homogenization of the exhaust-gas/fuel mixture is achieved. In this case too, it has proven advantageous to use an exhaust-gas part-stream and to preheat it, since, by way of example, injection of cold fuel into the main exhaust-gas stream would lead to incomplete vaporization of the fuel, with correspondingly poor homogenization of the mixture. The overall result of the method according to the invention is that the reformer is rapidly ready to operate and can supply hydrogen-containing reformer gas correspondingly quickly. After the reformer has started up, furthermore, preheated exhaust gas is fed to the reformer, which improves the heat balance of the latter and obviates the need for external heating.
In one configuration of the method, in order to heat the exhaust-gas part-stream the latter is passed through a separate exhaust-gas heater. The use of a separate, in particular electrically heated, exhaust-gas heater of this type allows the exhaust-gas part-stream to be heated up with little inertia and as required.
In one configuration of the method, in order to heat the exhaust-gas part-stream it is passed through an exhaust-gas heater and/or through a high-temperature heat exchanger through which the reformer gas flows. The gas is preferably guided by switching the gas path as required. When the reformer is being run up to its normal operating state, it is preferable for the exhaust-gas part-stream to be guided via the heated exhaust-gas heater. Once the reformer has been started up and is in its normal operating state, it supplies hot reformer gas at approximately 500xc2x0 C. or more. This heat content can be at least partially transferred to the exhaust-gas part-stream used by means of a high-temperature heat exchanger, with the result that the heat balance of the reforming process is improved. Depending on the size of the reformer-gas stream required and on the operating state of the reformer, it may be advantageous for the exhaust-gas part-stream to be additionally or exclusively passed through the exhaust-gas heater.
In a further configuration of the method, the exhaust-gas heater is electrically heated at least from time to time. The use of electrical energy for heating, for example by means of heating conductors or heater cartridges, results in advantages with regard to controllability and usability. The heating of the exhaust-gas heater preferably takes place when the reformer is starting up or at operating times when the reformer requires increased levels of heat, such as for example when there is an increased demand for reformer gas.
In a further configuration of the method, the exhaust-gas part-stream is passed through the heated exhaust-gas heater when the reformer is being started up. After the normal operating state of the reformer has been reached, the preheating of the exhaust-gas part-stream is effected predominantly by the high-temperature heat exchanger. For this purpose, the gas flow is switched over accordingly if necessary. This procedure makes it possible to make economical use of energy, since the heating of the exhaust-gas heater is predominantly switched off after the reformer has been started up.
In a further configuration of the method, the reformer-gas stream is fed to the exhaust gas upstream of the exhaust-gas catalytic converter at least from time to time when the exhaust gas from the internal combustion engine which is fed to the exhaust-gas catalytic converter has an excess of air. This preferably takes place after a cold start of the internal combustion engine and/or when the internal combustion engine is warming up, during which period it is operated with a lean air/fuel ratio. However, the internal combustion engine may also be operated with a rich air/fuel ratio during a cold start or while it is warming up, in which case secondary air is added to the exhaust gas upstream of the exhaust-gas catalytic converter, and as a result an exhaust gas with excess oxygen is established. In both cases, the result is that the exhaust-gas catalytic converter quickly reaches its operating temperature, since the oxidation of the reformer gas supplied, on account of its reactivity, takes place even when the exhaust-gas catalytic converter has only been heated to a relatively low temperature, and the heat of reaction which is liberated in the process rapidly heats the exhaust-gas catalytic converter. Of course, the supply of the reformer gas for heating the exhaust-gas catalytic converter may also take place during normal operation of the internal combustion engine, preferably if the exhaust-gas catalytic converter cools down excessively. This ensures that the catalytic converter function is maintained. The procedure described can be used with both a denox catalytic converter and an oxidation catalytic converter arranged in the exhaust pipe.
In a further configuration of the method, the reformer gas which is generated by the reformer is fed to the exhaust gas upstream of the exhaust-gas catalytic converter at least from time to time when a temperature on the entry side of the exhaust-gas catalytic converter or in the exhaust-gas catalytic converter falls below a predeterminable limit value. For this purpose, the temperature is expediently monitored or determined in some other way. In this way, the reformer gas can be added to the exhaust-gas catalytic converter according to demand.
Further features and combinations of features will emerge from the description and the drawings. For this purpose, the text which follows explains the invention in more detail with reference to drawings and associated examples.